Food Crops & EdiblesGreenhouse & Indoor Gardening

Why Too Much Sunlight Can Hurt Yields and Not Help Them

Let me describe my experiences on how sunlight has impacted tomato crop production experiments I have conducted or observed (Jones, 2008).

According to Wikipedia, the online dictionary, sunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. The Word Meteorological Organization defines sunshine as direct irradiance from the Sun measured on the ground of at least 120 watts per square meter. Direct sunlight has a luminous efficiency of about 93 lumens per watt of radiant flux, which includes infrared, visible, and ultra-violet light. Bright sunlight provides luminance of approximately 100,000 candela per square meter at the Earth’s surface. Sunlight is a key factor in photosynthesis, a process crucially important for life on Earth.

The 3 characteristics of sunlight that impact plants are intensity, day length, and wavelength composition. For several years, a colleague and I conducted a series of greenhouse tomato experiments, evaluating cultivar performance, and the effects of rooting media, plant population and topping on plant growth and fruit yield.

Identical experiments were conducted in two greenhouses, located a few miles apart, the one greenhouse was glass-covered, the other covered with fiberglass. Going from one greenhouse to the other, you would think that the experiments were completely different due to plant appearance. The tomato plants in the glass-covered greenhouse had a dull green foliage color with long internode stems, while in the fiberglass-covered greenhouse; the tomato plants had a lustrous dark green leaf color with short internode stems. Glass and fiberglass have different light transmitting characteristics, fiberglass filters out the long wavelength radiation, while glass the shorter blue wavelength light. Therefore, the differences in plant appearance were entirely due to wavelength composition of the transmitted sunlight.

A home gardener caught the attention of a local TV station when they learned of his unique growing system and high tomato fruit yields. During the interview, the grower gave the recipe for the rooting medium, a deep layer of a formulated mix of soil and composted organic matter. In addition, he credited much of his success to several other unusual procedures, such as placing a multi-vitamin tablet under each transplant when setting the plants. Watering was by means of a drip irrigation system, carefully regulated to keep the rooting medium moist, but not wet.

To keep rainfall off, the grower placed a heavy plastic roof cover over the plants. What both the grower and interviewer ignored was the filtering effect that the plastic cover was having on the incoming sunlight. Therefore, the high yield and fruit quality were primarily because of sunlight filtering, and not necessarily to the unusual characteristics of the rooting medium.

I experienced a similar effect when I was experimenting with a new hydroponic rooting system, using tomato as the test crop being grown in a double-walled plastic covered greenhouse. Since the plastic sheets covering the greenhouse were not airtight, both sheets had collapsed upon each other, trapping water between them. Obtained tomato fruit yields and quality were beyond my expectation. I assumed that my newly developed hydroponic growing system was the primary contributing factor. Not so! It was mainly due to the filtering of the sunlight passing through the thin water film trapped between the two sheets of plastic. No doubt the new hydroponic rooting system may have contributed to the high fruit yield and quality obtained, but it was not the primary one.

Several years ago, I consulted with 4 commercial greenhouse tomato growers whose fruit yields and quality of fruit that year were lower than what they had obtained in previous years. My repeated visits didn’t uncover what might be the possible cause(s) until one grower remarked that measured incoming radiation was considerably higher that year than what he had recorded in previous years. Being an engineer, this grower kept daily records on greenhouse environmental conditions, including noontime measurement of photosynthetically active radiation (PAR) using a handheld PAR-recording meter. That was the clue that led me to obtain weather station measurements of solar radiation as minutes of daily sunshine.

Collecting several years of data and correlating it with fruit yields obtained by these 4 growers, I concluded that if monthly minutes of sunshine exceeds 10,000, then fruit yield and quality will be poor (see Jones and Gibson, 2001). Later the grower using his PAR meter supplied me weekly minutes of sunshine data so that it could be correlated with weekly tomato fruit yield. There was no correlation between the two values for the same week. However, there was a correlation between minutes of sunshine 3 weeks prior to the weekly fruit harvest, but the correlation was negative, meaning that a high sunshine week resulted in a lower fruit yield 3 weeks later!

Such a result may not apply at all latitudes since these observations were made in South Carolina whose latitude is approximately 35oN. From this experience, I now recommend installing moveable shade in the greenhouse so that it can be pulled over the plant canopy during periods of high radiation intensity – mainly during the high noon hours.

Being an avid vegetable gardener, I have been growing vegetables, tomato being one, outdoors hydroponically. For several years, the growing site was one that received full morning sun but was in partial shade in the afternoon due to the presence of nearby tall trees. A neighbor was using one of my hydroponic boxes containing 4 tomato plants, the same growing unit that I had at my garden site. However, my neighbor’s site was shaded in the morning, but by noon, was in full sun the rest of the day. The plant-support stakes in the growing boxes were 5 feet. When the tomato plants reached the top of the stakes, I topped the plants, letting suckers develop with the hope that additional fruit would set and mature on these suckers.

To my surprise, it worked! I had another 6 weeks of tomato fruit production at my site. But for my neighbor, it didn’t work as the suckers grew slowly and not a single fruit was set! Therefore, my recommendation for successful tomato fruit production is to select a site that receives full morning sun and then is in partial shade during the afternoon hours. Again, being in South Carolina, this effect may not apply to those gardening at a higher latitude.

Dr. J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books. Dr. Jones has written extensively on hydroponic growing and outdoor vegetable gardening employing sub-irrigation hydroponic growing systems.


Jones, Jr. J.B.  2008.  Tomato Plant Culture:  In the Field, Greenhouse, and Home Garden.  CRC Press, Boca Raton, FL.

Jones, Jr., J.B. and P.A. Gibson.  2001.  Excessive solar radiation and tomatoes.  Growing Edge 12(6):60-65.

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